1. Field of the Invention
The present invention relates to an apparatus and method of operating a recording medium, and more particularly, to an apparatus and method of removing a disturbance in a recording medium having a large eccentricity and deflection.
2. Description of the Related Art
The eccentric mass of a disc in an optical recording medium drive system is caused by a discrepancy between a spindle rotation axis for rotating a disc, the center of a track of the disc and different curvature characteristics of the optical recording medium. Further, the distance between a pickup and an optical recording medium may change according to the shape of the recording medium, or because its surface shakes as the recording medium rotates. In particular, such a change occurs when the surface of the optical recording medium vibrates in the vertical direction of the optical recording medium, which is referred to as “deflection”.
The eccentricity and deflection of the optical recording medium are major factors causing a disturbance in an optical recording medium drive system, and clearly affect the recording and reproducing capability of the optical recording medium drive system at a high playback speed. Therefore, the eccentricity and deflection must be compensated in order to precisely follow a track. In the past, the eccentricity of an optical recording medium has been compensated using a learning control algorithm.
FIG. 1 is a block diagram of a tracking control system including a conventional repetition learning control structure. This tracking control system includes a sensor K(S) 10, a controller C(S) 11, a driver V(S) 12, an actuator P(S) 13, and a repetition learning controller 14. The sensor K(S) 10 outputs a tracking error signal e(t) containing information on the center of a track and the position of a pickup. The sensor K(S) 10 includes a photo diode (not shown) that detects the extent to which a pickup (not shown) deviates from a pre-focus position or the center of a track, and a radio-frequency (RF) amplifier (not shown) that amplifies a signal output from the photo diode and outputs the tracking error signal e(t). The controller C(S) 11 receives the tracking error signal e(t) output from the sensor K(S), and outputs a compensation signal instructing the pickup to be positioned at the center of a track even if a disturbance of the track occurs. The compensation signal output from the controller C(S) 11 is applied to the actuator P(S) 13 via the driver V(S) 12.
In an optical recording medium drive system, disturbances occur for many reasons. For example, noise causes a disturbance. However, a main factor in causing disturbances is the eccentricity of a disc. The disturbance due to the eccentricity of a disc is caused by the rotation of the disc, and occurs periodically in accordance with the disc rotation frequency. The disturbance due to eccentricity contributes to the tracking error. In general, an increase in the eccentricity mass of a disc results in an increase in the size of a periodic disturbance component of the tracking error. Thus it is difficult to compensate for such a periodic disturbance with only the controller C(S) 11. For this reason, a repetition learning control is performed by the repetition learning controller 14 so as to cancel the periodic disturbance.
The repetition learning controller 14 reads an output Ufb(t) of the controller C(S) 11 in response to an FG signal, which is a synchronization signal used to judge a period, and stores data Uff(t), which is to be compensated in a memory 14-2 according to a learning control algorithm 14-1. Once the compensation of the data Uff(t) is completed according to the learning control algorithm 14-1, the data Uff(t) is combined with the output Ufb(t) to produce a signal U(t) which is applied to the actuator P(S) 13. As a result, the periodic disturbance due to the eccentricity of the disc is canceled. The performance of the repetition learning controller 14 depends on how much data is stored in the memory 14-2 during a rotation period of a disc.
There are two main methods of learning data using the repetition learning controller 14. One method is to store an output of the controller C(S) 11 in the memory 14-2 during one period of the FG signal, and output the data stored in the memory 14-2 in synchronization with the FG signal during the next period. An advantage of this method is that learning time is short, but the method can only be used on the assumption that the controller C(S) 11 is operating normally.
The other method is to repeatedly learn data for several periods until the size of a tracking error is reduced by a certain degree. In detail, the data Uff(t) output from the learning control algorithm 14-1 is repeatedly learned in consideration of an output of repetition learning performed during the previous period, and a tracking error. Here, the data is continuously learned over several periods. Therefore, the more periods over which the learning is repeated, the more the periodic disturbance contained in a tracking error can be reduced. That is, continuously learning a periodic disturbance makes it possible to cancel as much of the periodic disturbance as possible. However, this method requires a lot of time for repetition learning.
Meanwhile, a repetition learning algorithm uses an FG signal so as to synchronize period. Here, the FG signal has a three or six period according to the phase of a spindle motor during which a disc is rotated for one period. During one rotation period of a disc, the less the number of FG signals, the less the number of FG edges through which synchronization of a period is checked. Thus, the performance of the repetition learning algorithm 14-1 is lowered. In general, the performance of the repetition learning algorithm 14-1 depends on how much data can be stored and output during one rotation period of a disc. However, if the amount of data to be stored is increased, the sampling frequency increases and the capacity of the memory 14-2 in which the data is stored must be increased. On the other hand, if the amount of data to be stored is small, the resolution of the repetition learning controller 14 becomes poor and the performance of the repetition learning algorithm 14-1 deteriorates.
Further, a high performance microcontroller (not shown) or a programmable digital signal processor (DSP) (not shown) is required to apply a repetition learning algorithm 14-1 to a conventional tracking control system. However, in fact, most optical recording medium drive systems use a hard-wired DSP and an inexpensive 8-bit microcontroller. Therefore, a repetition learning algorithm 14-1, which places a burden on the system, is not available.